1. Field of the Invention
The invention relates to storage stable porfimer sodium compositions useful in photodynamic therapy and improved methods for their manufacture.
2. Description of the Related Art
The use of porphyrin compounds, and in particular hematoporphyrin and its derivative mixture hematoporphyrin derivative (HPD), have been known for some time to be useful systemically when combined with irradiation, for the treatment and diagnosis of malignant cells. The porphyrins appear to "naturally" localize in malignant tissue where they absorb light at certain wavelengths when irradiated, providing a means to detect the tumor by the location of the fluorescence. Accordingly, preparations containing the porphyrins are useful in the diagnosis and detection of such tumor tissues. (See e.g. "Porphyrin Photosensitization", Kessel, D., et al., eds. (1983) Plenum Press). In addition, the porphyrins also have the capability of exhibiting a cytotoxic effect on the cells or other tissues in which they are localized when irradiated at the appropriate wavelength. (See, e.g., Diamond, I., et al., Lancet (1972) 2: 1175-1177; Dougherty, T. J. et al., "The Science of Photo Medicine" (1982) J. D. Regan & J. A. Parrish, eds., pp. 625-638; Dougherty, T. J., et al., "Cancer: Principles and Practice of Oncology" (1982) V. T. DeVita Jr., et al., eds., pp. 1836-1844). It has been postulated that the cytotoxic effect of the porphyrins is due to the formation of singlet oxygen upon irradiation (Weishaupt, K. R., et al., Cancer Research, (1976) 36: 2326-2329). The successful treatment of AIDS-related oral Kaposi's Sarcoma with a purified form of HPD, Photofrin.RTM. porfimer sodium, was described in Schweitzer, V. G. et al., Otolaryngology--Head and Neck Surgery, (1990) 102: 639-649.
In addition to systemic use for the treatment and diagnosis of tumors, the porphyrins can be used in a variety of other therapeutic applications. For example, photosensitizers are useful in the detection and treatment of artherosclerotic plaques as disclosed in U.S. Pat. Nos. 4,517,762 and 4,577,636. U.S. Pat. Nos. 4,500,507 and 4,485,806 describe the use of radiolabeled porphyrin compounds for tumor imaging. Porphyrin compounds have also been used topically to treat various skin diseases as disclosed in U.S. Pat. No. 4,753,958.
A number of porphyrin photosensitizer preparations have been disclosed for therapeutic applications. A photosensitizer preparation widely used in the early stages of photodynamic therapy both for detection and treatment was a crude derivative of hematoporphyrin, also called hematoporphyrin derivative (HPD) or Lipson derivative, prepared as described by Lipson et al., J. Natl. Cancer Inst. (1961) 26: 1-8. A purified form of the active component(s) of HPD was prepared by Dougherty and co-workers by adjustment of the pH to cause aggregation and recovery of the aggregate, as disclosed in U.S. Pat. Nos. 4,649,151, 4,866,168, 4,889,129 and 4,932,934. A purified form of this product has been used clinically under the trademark Photofrin.RTM. porfimer sodium (also referred to as polyhematoporphyrin ether/esters).
Porfimer sodium is prepared by following the method of Lipson as modified by Dougherty et al. In brief, hematoporphyrin is first treated with a mixture of sulfuric and acetic acids which produces a mixture of mono- and di-acetate of hematoporphyrin (Hp) and its dehydration products, hydroxyethylvinyldeutero porphyrin (Hvd) and protoporphyrin. This mixture upon treatment with 0.1 N NaOH undergoes hydrolysis and coupling to produce hematoporphyrin derivative (Hpd) which is a mixture of Hp, Hvd, protoporphyrin and a higher molecular weight fraction consisting of a mixture of porphyrin oligomers. In the Dougherty et al. disclosure, this higher molecular weight fraction was separated by adjustment of the pH to around 9.5 and removal of the resulting aggregate by membrane filtration. See U.S. Pat. No. 4,649,151.
The structure of porfimer sodium and the nature of linkages joining the porphyrin units has been the subject of discussion for quite some time. Bonnett and Berenbaum, Adv. Exp. Biol. Med. 160, 241 (1983) proposed that the structure of the tumor localizing component might be a dimer or oligomer linked by ether or carbon to carbon covalent bonds. Dougherty et al., Prog. Clin. Biol. Res., (1984) 170: 301-314, presented evidence from fast atom bombardment mass spectroscopy (FAB-MS) and NMR spectroscopy that the localizing components were most likely the isomers of dihematoporphyrin ether. Recently Kessel and coworkers, Cancer Res. 47, 4642 (1987) have suggested that the tumor localizing components of porfimer sodium might be porphyrin dimers with ester and ether linkages. However, the presence of a polymeric mixture was not ruled out. Recently it has been reported that hematoporphyrin dimer, trimer and their dehydration products with ether linkages are biologically active and are likely active components in porfimer sodium, Pandey et al., Tetrahedron Lett. 29: 4567 (1988). Hematoporphyrin dimer with ester linkage was found to be biologically inactive and is only a minor component of porfimer sodium, Pandey et al., Cancer Res. 49: (8) 2042-2047 (1989). It is now believed, from fast atom bombardment mass spectral analysis of porfimer sodium, that it is in fact a complex mixture of oligomers having from two up to eight porphyrin units linked together and that the porphyrin units are hematoporphyrin, hydroxyethylvinyl-deuteroporphyrin and protoporphyrin units. The porphyrin units are linked with ether and/or ester bonds as generally exemplified in structure I, below: ##STR1## In the foregoing structures, n is an integer from 0 to 6 and R is hydroxyethyl or vinyl. It is understood that the composition includes a combination of oligomers of varying chain length, that is, a combination of compounds where n is 0, 1, 2, 3, 4, 5 or 6 (dimers, trimers, tetramers, etc.). In addition, the composition includes a combination of hematoporphyrin, hydroxyethylvinyl-deuteroporphyrin, and protoporphyrin units (i.e. R is hydroxyethyl and/or vinyl).
The porphyrin units may be linked together in different ring orientations, depending on whether ether or ester bonds are involved. Referring to the structure below: ##STR2## where the rings of the porphyrin are labelled A, B, C and D, the ether linked porphyrins may be joined only with A-A, A-B, B-A, or B-B orientations. Ester linked porphyrins may be joined with the A-C, A-D, B-C or B-D orientations.
Heretofore, the use of porfimer sodium as a therapeutic or diagnostic agent has been hindered by the relative instability of the product. Porfimer sodium prepared by the prior art process degrades rapidly when exposed to heat and is stable for only a few hours at room temperature. Accordingly, the solution is generally kept frozen and thawed immediately prior to use.
The use of such a frozen solution has many obvious disadvantages. Because it has to be kept frozen, it must be shipped and stored in a frozen state, necessitating the use of special freezing conditions. For example, the product must be shipped in special containers using dry ice or the like as a refrigerant. This is a major drawback, adding to the cost and logistics of using the product. At the point of use, the frozen solution must be stored at -20.degree. C., which is below the operating temperatures of some freezers thereby necessitating special freezer equipment. In addition, the frozen product must undergo a thawing period and is therefore not useable immediately with a patient.
Various methods have been proposed to overcome that instability of the product. U.S. Pat. No. 5,059,619 discloses a stable, freeze-dried preparation of porfimer sodium which is obtained by freeze-drying an aqueous solution of the photosensitizer composition. This preparation is reconstituted prior to administration. Lai et al., U.S. Pat. No. 4,882,234, discloses a storage stable composition which is obtained by heat treating a buffered pH 5-9 aqueous solution of hematoporphyrin at a temperature of 60.degree. C. to about 120.degree. C. for 5-100 hours. The resulting product appears to have a different composition than porfimer sodium and is stated to contain at least 80% by weight of its porphyrins as ester linked oligomers where the oligomeric chain is an average of about 3 to 7.
It is an object of the present invention to provide porfimer sodium compositions which exhibit enhanced thermal and temporal stability as compared to preparations shown in the art, and which maintain high levels of activity as photodynamic therapeutic and diagnostic agents. It is a further object of this invention to provide processes for producing such materials and methods of using such materials.